| AVS 57th International Symposium & Exhibition | |
| Applied Surface Science | Tuesday Sessions |
| Session AS-TuM |
| Session: | Electron Spectroscopies |
| Presenter: | S. Techane, University of Washington |
| Authors: | S. Techane, University of Washington L. Baugh, University of Washington L.J. Gamble, University of Washington P.S. Stayton, University of Washington D.R. Baer, Pacific Northwest National Laboratory D.G. Castner, University of Washington |
| Correspondent: | Click to Email |
The purpose of this research is to study the interactions of biomolecules with nanoparticles using model systems. Model proteins and nanoparticle surfaces are used to simplify the complex biological/nanoparticle surface environment and develop a fundamental understanding of the interactions at this interface. Protein G B1 domain was used as a model protein. Wild type Protein G B1 (WT) has a uniform distribution of negatively charged residues on its surface. Using site-directed mutagenesis to neutralize six charged residues at one end of the protein, a mutant (Δ6) was created possessing a global charge dipole along its long axis. This variation in surface charge affected the biomolecular interaction with nanoparticle surfaces. Gold nanoparticles (AuNPs) with self-assembled monolayers of carboxylic alkanethiols (COOH-SAM) were used as the well-defined model surface.
We previously reported that AuNPs 14nm in diameter and functionalized with C16 COOH-SAM were more monodisperse in size and shape relative to those with larger diameter, and more stable in solution relative to those functionalized with shorter-chain COOH-SAMs. To accurately interpret XPS data and account for the curvature effect, and to quantitatively analyze overlayer thickness (T) and roughness (R), we used SESSA combined with geometric correction methods. The C16 COOH-SAM on flat Au was modeled with SESSA at photoelectron take-off angles from 5 to 85°. Corresponding ARXPS experiments were also performed. T and R were optimized to find the best agreement between the simulated and experimental results. It was necessary to add a hydrocarbon contamination layer (CL) at the surface of the COOH-SAMs in the simulations. For the C16 COOH-SAM on flat Au, the best conditions were T=1.1Å/CH2, R=1.05 and CL=1.5Å. After applying a geometric correction, T=0.9Å/CH2 and R=1.05 was obtained for the SAM on AuNPs with similar CL. Studies to minimize CL are underway.
WT and Δ6 B1-protein G were adsorbed onto 14nm-AuNP-C16 COOH-SAM at various protein concentrations. Following adsorption and centrifugation, the final supernatant containing unbound protein and AuNPs with bound protein were analyzed by colorimetric assay and XPS, respectively. Both methods showed that Δ6 adsorbed more than WT. As Δ6 concentration increased, the N signal (unique to the protein) increased and the Au signal (unique to the surface) decreased, following similar trends as observed in the adsorption isotherm curve. Based on our preliminary study, a %N of 13.6 ± 0.3 corresponded to a monolayer of ~0.95μg/cm2. Further studies to accurately determine thermodynamic adsorption parameters and the AuNP concentration are underway.